Conventionally, various measuring apparatuses have been used for recognizing a concentration of a desired gas component in a measurement gas. For example, as a device for measuring a NOx concentration in a measurement gas such as a combustion gas, known is a gas sensor (NOx sensor) having oxygen-ion conductivity, such as one made constituted by zirconia (ZrO2) (for example, see Patent Document 1). In such a gas sensor, in general, a sensor element has a plurality of electrode terminals formed on a surface thereof for applying a voltage, retrieving a detection signal, supplying power to a heater part, and the like.
Here, the gas sensor includes a contact member for holding the sensor element inserted therein. For example, a gas sensor is already known having a contact member that includes: a housing in which an insertion port for insertion of a sensor element is defined by a pair of housing members arranged opposed to each other; a plurality of contact-point members configured as metal terminals provided on the housing member; and a plurality of lead wires connected to the contact-point members for electrical conduction between the sensor element and the outside (for example, see Patent Document 2).
In the gas sensor disclosed in the Patent Document 2, the contact member holds the sensor element inserted in the insertion port while making the contact-point members and electrode terminals in contact with each other, thereby obtaining electrical conduction between the sensor element and the outside. In other words, in the gas sensor disclosed in the Patent Document 2, the contact-point members serve as contact points with the electrode terminals. Specifically, in a state where the sensor element is inserted in the insertion port, the housing members are fitted into a fixture having a substantially concave shape (or substantially C-like shape) in a cross-sectional view and having pressure springs formed at upper and lower sides thereof, and additionally a clamping ring is arranged at an outer circumferences of the fixture and the pressure springs and then the clamping ring is clamped so that the pressure springs are displaced to bias the contact-point members to the electrode terminals due to an elastic force of the pressure springs, thus holding the sensor element and ensuring the electrical conduction.
Here, the fixture disclosed in the. Patent Document 2 not only serves to fix a spring member as described above but also serves to restrain the pair of housing members in a predetermined positional relationship until the sensor element is inserted in the insertion port and the clamping ring is clamped. This restraint is necessary for achieving a correct contact between each electrode terminal and a corresponding contact-point member without misalignment at the time of holding and fixing the sensor element between the pair of housing members (that is, in the insertion port) by clamping the clamping ring. In a case of the Patent Document 2, since the fixture has a concave shape in a cross-sectional view, an elasticity of the fixture serving as a leaf spring is utilized for holding and restraining the housing members.
In the gas sensor disclosed in the Patent Document 2, the clamping ring is clamped so that the elastic force of the pressure springs is applied to the housing via the fixture, thereby fixing the sensor element to the housing. At this time, the contact-point members provided on the housing bias the electrode terminals, and thereby the contact between the electrode terminals and the contact-point members is ensured.
Here, however, a problem arises that a force acting on the housing may be asymmetric (non-uniform) because the fixture has a concave shape in a cross-sectional view as described above. In a gas sensor configured to have a plurality of electrode terminals in contact with corresponding contact-point members, respectively, a biasing force of the contact-point member to the electrode terminal varies depending on positions due to the non-uniformity, which may consequently cause a local contact failure.